The screw pump is a positive displacement rotor pump, born in 1931. Because of its unique structure, self-priming ability, high efficiency, small size, reliable work, and can transport various media with a wide range of viscosity, screw pumps are widely used in petrochemical, shipping, electric power, mechanical hydraulic systems, food, papermaking, Industrial sectors such as sewage treatment.
As an energy-saving and material-saving product, the scope of application of screw pumps in our country is constantly expanding, and the demand has been increasing year after year, which has attracted more and more attention. Although there is still a gap between my country’s domestic screw pumps and foreign professional companies, with the increase in scientific research and application practices of my country’s screw pumps, screw pumps will surely achieve good results in replacing the original traditional technology and realize energy saving. , Material saving benefits.
Basic knowledge of screw pump
Although the screw pump family is not huge, it can be divided into different types according to the screw standard. This article focuses on the most commonly used single-screw pumps, twin-screw pumps and three-screw pumps.
1, single screw pump:
The single screw pump is a new type of internal meshing rotary positive displacement pump. The main working parts are eccentric screw (rotor) and fixed bushing (stator).
Compared with other pumps, the single screw pump has its own unique advantages:
Compared with centrifugal pumps, single screw pumps do not need to be equipped with valves, and the flow is a stable linear flow.
Compared with plunger pump, single screw pump has better self-priming ability.
Compared with diaphragm pumps, single screw pumps can transport various mixed impurities, media containing gases and solid particles or fibers, and can also transport various corrosive substances.
Compared with gear pumps, single screw pumps can transport high-viscosity materials.
Different from plunger pump, diaphragm pump and gear pump, single screw pump can be used for medicine filling and metering.
2. Twin screw pump:
In terms of structure, the twin screw pump is an externally intermeshing screw pump. It uses two screws that mesh with each other and do not touch each other to pump liquid. In terms of structure, the twin-screw pumps are also very complete. There are various types such as horizontal, vertical, and heating jackets. They can transport low-viscosity or high-viscosity media with particles. Adjust the screw spacing according to the particle size and select the correct material. It can even transport many corrosive media.
Twin screw pump is a positive displacement pump, the suction chamber in the pump should be tightly separated from the discharge chamber. This requires that the outer surface of the pump body and the screw and the gap between the screw and the screw should be as small as possible. At the same time, the screw and the pump body and the screw and the screw form a sealed cavity with each other to ensure airtightness, otherwise there may be liquid flowing back from the gap.
The unique structure of the twin-screw pump enables it to deliver various media without stirring, pulsation, and smoothness; because the pump body structure ensures that the pumping liquid is always stored in the working elements of the pump as a sealing liquid, the twin-screw pump is very strong The self-priming ability, and can mix vapor and liquid. The special design of the twin screw pump also ensures that the pump has a high suction performance, that is, a small NPSHr value.
Twin screw pumps can be divided into two types: internal bearing and external bearing. In the structure of the built-in bearing, the bearing is lubricated by the conveyed material. For twin-screw pumps with an external bearing structure, the working chamber is separated from the bearing. There is a side gap between the structure of this pump and the screw, and the independently lubricated external bearing allows it to transport various non-lubricating media. In addition, adjust the synchronous gear so that the screw does not contact, and at the same time transfer half of the output torque to the driven screw. Like all screw pumps, the external bearing type twin screw pumps also have self-priming capability, and most of the pump delivery elements themselves are double-suction symmetrically arranged, which can eliminate axial force and have a large suction height.
The advantages of the twin-screw pump structure with external bearings greatly broaden the scope of use of the twin-screw pump, that is: in addition to transporting good lubricating media, it can also transport a large number of non-lubricating media with various viscosities (the highest viscosity can reach 3 *106mm/s) medium and corrosive (acid, alkali, etc.) and abrasive liquids; in addition, the twin screw pump is a non-sealed positive displacement pump due to its constant gap and the characteristics of the molding line. In addition to transporting pure liquids, it can also transport gas and liquid mixtures, that is, vapor-liquid mixed transport. This is also one of the unique advantages of twin-screw pumps; twin-screw pumps can also run dry: because the moving parts do not touch each other during work Therefore, the short-term dry running will not damage the pump components. This feature provides great convenience to the automatic control process, but the dry running time is limited by many factors and is generally very short. In addition, the twin-screw pump has no shear and emulsification during the conveying process, so it will not destroy the molecular chain structure and the specific fluid properties formed in the working condition process, and because the transmission relies on synchronous gears, the pump runs with low noise and low vibration. , Work smoothly.
The various characteristics of the external bearing twin screw pump make it widely used in the oil field chemical industry and shipbuilding industry. It can also be made of different materials such as ordinary cast iron and stainless steel according to various usage conditions; the conveying temperature can reach 250℃; and it has different heating structures, and the theoretical flow rate can reach 2000 cubic meters per hour.
3. Three screw pump:
The three-screw pump is a rotor-type positive displacement pump. It uses the principle of screw meshing and relies on the rotating screw to mesh with each other in the pump casing.
It is mainly composed of a bush (pump cylinder) fixed in the pump body, a driving screw inserted in the pump cylinder and two driven screws meshing with it. The three intermeshing screws form a sealed cavity according to each lead in the pump cylinder, resulting in a seal between the suction and discharge ports.
The three-screw pump seals the conveyed medium in the meshing cavity, and pushes it to the discharge port continuously and uniformly along the screw axis to provide a stable pressure for the system. When working, the suction and discharge pressures of the liquid are acted on at both ends to generate axial thrust on the screw. In addition, high-pressure oil is introduced into the bottom of each screw sleeve through the central oil hole of the driving screw, so that a balanced thrust opposite to the axial thrust will be generated at the lower end of the screw. Therefore, for small pumps with a pressure difference of less than 10 kgf/cm2, thrust bearings can be used.
Since the two driven screws mesh with the driving screw symmetrically, the radial force acting on the driving screw of the three-screw pump is completely balanced, and the driving screw does not bear the bending load. The radial force of the driven screw is supported by the pump cylinder bushing along its entire length, no additional bearing is required at the outer end, and basically it does not bear the bending load. During operation, an oil film formed between the outer surface of the screw and the inner wall of the pump cylinder can prevent direct contact between metals and greatly reduce the wear of the screw tooth surface.
The three-screw pump is the same as other positive displacement pumps. When the discharge port of the pump is completely closed, the pressure in the pump will rise to the dangerous degree of damage to the pump or overload of the motor. Therefore, a safety valve must be installed at the suction and discharge port of the pump. The shaft seal usually adopts a mechanical shaft seal, and it can take different forms according to the working pressure.
The remarkable characteristics of the three-screw pump are simple structure, small pressure pulsation, stable flow, stable and reliable operation, high speed, low noise, high efficiency, long life, and self-priming ability. According to the needs of the conveyed medium, the screw pump device can provide heating or cooling structure. It is suitable for conveying various non-corrosive oils and similar oils and lubricating liquids. The viscosity range of the conveyed liquid is generally 3.0~760mm2/s (1.2~100°E). High-viscosity media can be conveyed after heating and reducing its viscosity, and its temperature generally does not exceed 150°C. The flow range is generally 0.2~250m3/h, and the maximum working pressure can reach 4MPa.
At present, three-screw pumps are widely used in petroleum, chemical fiber, metallurgy, machinery, electric power, machine tools, ships, glass, highways and other industries. The three-screw pump is suitable for transporting fuel oil, lubricating oil, hydraulic oil, crude oil, asphalt and other lubricating liquids like oil. The three-screw pump is used as a lubrication pump in the industrial field, as a hydraulic pump in a hydraulic system, as a delivery and booster pump in a fuel system, and as a delivery and refueling pump in a fuel delivery system.
